This invention relates to peelable bonded structures and processes for preparing them. More particularly, it relates to a bonded structure in which a metal substrate and a polyolefin layer are tightly bonded together in a specific region (bonded region), and in the region (easily peelable region) other than said bonded region, the metal substrate and polyolefin layer are maintained in the easily peelable bonded state or non-bonded state.
Bonded structures comprising a plurality of articles which are bonded together to such an extent that they can be peeled from each other are often required in various fields, especially in the packaging of materials involving vessels, vessel closures and sealing materials.
Sealing materials such as crown closures and other vessel closures, have heretofore been prepared by coating a surface-protecting paint onto a metal sheet, forming the coated metal sheet into a crown shell, cap shell or the like and bonding a packing material to the inner face of the formed article. In premium sales of bottled beverages and the like, systems have been adopted in which a prize or premium is given for a predetermined number of packings or a packing having a winning mark. In the production of crown closures or caps for use in such premium sales, it is first of all required that packings be easily peelable from crown shells or cap shells. However, it also is required that packings be bonded to crown shells or cap shells to such an extent that the packings are not peeled during the preparation of such crown closures or caps, during their transportation or during the step of sealing the bottles. Also, crown or cap shells should have adequate corrosion resistance to the contents of the vessel and they should be able to withstand such processing as crimping or roll-on. Also, from the sanitary viewpoint, it is not permissable to print the surface of a packing which directly contacts a beverage. In general, a packing per se is applied to the inner face of a crown shell or cap in the state where it has a certain flowability. Accordingly, it is desirable that when a packing is peeled off, a printing ink layer formed on the inner face of a crown shell or cap is transferred in the state adhering to the packing.
These special peeling characteristics are also required for cans having so-called "easy-open"-type mechanisms. For example, a can vessel having an easy-open end, which is prepared by forming one or more openings necessary for drinking through a can end-member composed of a surface-protecting coating-applied metal sheet and bonding a peel piece composed of an organic resin-coated metal foil or sheet over said opening or openings, is known in the art. Also, in such easy-open end structures, excellent adhesion and sealing are required between the can end member and peel piece during storage, but when the can vessel is opened, the peel piece should be easily peelable from the can end member without risk of breaking the peel piece.
In order to form a bonded structure of two articles which can be peeled from each other in the bonded area without breakage of the articles, it is necessary that the peel strength of the bonded region should be within a certain range, generally about 0.02 to 5 kg/cm. If the peel strength is lower than 0.02 kg/cm, peeling is undesirably found to occur before the articles are intentionally peeled. On the other hand, when the peel strength is greater than 5 kg/cm, it is generally difficult or impossible to effect peeling at the bonded interface without breaking the bonded articles.
Furthermore, in order to easily peel two bonded articles from each other at their bonded interface, it is necessary that peeling be facilitated in that portion of the bonded interface where peeling is initiated.
Accordingly, it is an object of the present invention to provide a novel peelable bonded structure comprising a metal substrate and a polyolefin layer bonded to the metal substrate such that the bond strength between the substrate and the polyolefin layer varies at predetermined regions with respect to the plane direction of the bonded structure.
Another object of the invention is to provide a peelable bonded structure in which a metal substrate and a polyolefin layer are bonded together to such an extent that the bonded structure as a whole has a peel strength of between about 0.02 and 5 kg/cm or so that interlaminar peeling can easily occur between the metal substrate and the polyolefin layer in the peeling-initiating region.
Another object of the invention is to provide a vessel closure comprising a metal substrate such as a crown shell or cap shell and a polyolefin packing material peelably bonded to the metal substrate.
Another object of the invention is to provide an easy-open end structure comprising a can end member having an opening and a peel piece peelably bonded to the can end member.
Yet another object of the invention is to provide a process for producing the aforementioned peelable bonded structures.
These and other objects of the invention as well as a fuller understanding of the advantages thereof can be had by reference to the following detailed description, drawings and claims.